Testosterone is one of the hormones in men that spur the development of secondary sexual characteristics. Testosterone is produced by the testes in response to certain hormonal and nervous system stimulation. Testosterone production typically peaks in the late teens and early twenties and gradually declines with age. It has been estimated that one third of men over the age of 45 years of age have low testosterone levels.
Normally, a male's total testosterone blood level is in the range of about 270 ng/dL to about 1070 ng/dL (about 9 nmol/L to about 38 nmol/L), with free testosterone blood levels falling in the range of about 50 pg/mL to about 210 pg/mL (about 174 pmol/L to about 729 pmol/L). Levels below these ranges indicate clinically low testosterone levels. Individuals having lower testosterone levels, but not low enough to result in a clinical diagnosis of a low testosterone level may also suffer from symptoms associated with the lower level.
The symptoms of low testosterone levels in males include decreased libido, erectile dysfunction, loss of body and/or facial hair, weakened bones, increased body fat, and fatigue. Currently, subjects with symptoms of low testosterone levels are treated with hormone replacement therapy such as Androgel® and Testim®. Testosterone replacements are typically applied transdermally or transmucosally and can have a number of side effects including rash, itching, or irritation at the site where the testosterone is applied. There is also the potential for benign prostatic hypertrophy, prostate cancer, sleep apnea, erythrocytosis, and congestive heart failure with these hormone replacement therapies. Methods of safely increasing the endogenous production of testosterone are needed that may avoid some or all of the potential side effects of testosterone hormone replacement therapies.